1. Field of the Invention
This invention relates to a method for the production of an absorbent resin. More particularly, it relates to a method for the production of an absorbent resin which exhibits high level of absorption capacity under load as well as absorption capacity without load and, when used in a sanitary material, manifests a particularly excellent quality.
2. Description of the Prior Art
Absorbent resins have been finding extensive utility in recent years as one of the component substances for such sanitary materials as disposable diapers and sanitary napkins which are intended for the absorption of bodily humors.
The absorbent resins which have been known in the art include partially neutralized cross-linked polyacrylic acids (JP-A-SHO-55-84,304, JP-A-SHO-55-108,407, and JP-A-SHO-55-133,413), hydrolyzates of starch-acrylonitrile graft polymers (JP-A-SHO-46-43,995), neutralized starch-acrylic acid graft polymer (JP-A-SHO-51-125,468), saponified vinyl acetate-acrylic ester copolymers (JP-A-SHO-52-14,689), hydrolyzed acrylonitrile copolymers or acrylamide copolymers (JP-A-SHO-53-15,959) or cross-linked products of such hydrolyzates, and cross-linked cationic monomers (JP-A-SHO-58-154,709 and JP-A-SHO-58-154,710), for example.
The absorbent resins are desired to excel in such characteristic properties as the absorption capacity, the absorption rate, the liquid permeability, the gel strength and the suction power to absorb liquid from a substrate containing the aqueous liquid, for example. These characteristic properties, however, do not necessarily show a positive correlation. Such physical properties as liquid permeability, gel strength, and absorption rate, for example, decline in accordance with the increase of absorption capacity.
As means for conferring an ideally balanced improvement on the various absorption properties of an absorbent resin, the technique of cross-linking the surface region of the absorbent resin has been known to the art. Various methods for working this technique have been proposed to date.
For example, as a cross-linking agent, the method using a polyhydric alcohol (JP-A-SHO-58-180,233 and JP-A-SHO-61-16,903), the method using a polyglycidyl compound, a polyaziridine compound, a polyamine compound, or a polyisocyanate compound (JP-A-SHO-59-189,103), the method using glyoxal (JP-A-SHO-52-117,393), the method using a polyvalent metallic element (JP-A-SHO-51-136,588, JP-A-SHO-61-257,235, and JP-A-SHO-62-7,745, the method using a silane coupling agent (JP-A-SHO-61-211,305, JP-A-SHO-61-252,212, and JP-A-SHO-61-264,006), the method using an epoxy compound and a hydroxy compound (JP-A-HEI-2-132,103), and the method using an alkylene carbonate (DE-4020780) have been known in the art. Besides these methods, during the cross-linking reaction, the method using the presence of an inert inorganic powder (JP-A-SHO-60-163,956 and JP-A-SHO-60-255,814), the method using the presence of a dihydric alcohol (JP-A-HEI-1-292,004), the method using the presence of water and an ether compound (JP-A-HEI-2-153,903), etc. have been also known.
These methods indeed bring about a perceptibly balanced improvement of the physical properties of an absorbent resin. The improvement, however, can hardly be called sufficient. The absorbent resins are still in need of a further improvement in quality. Particularly in recent years, the desirability of an absorbent resin which retains at a high level the absorption capacity without load, one of the basic physical properties of the conventional absorbent resin, and meanwhile excels in absorption properties under load, particularly the absorption capacity under load, has come to find growing recognition. Naturally, the absorption capacity without load and the absorption capacity under load are generally in a contradictory relation. In fact, the heretofore known techniques for cross-linking the surface region of an absorbent resin are such that they cannot fully satisfy this desirability.
It is, therefore, an object of this invention to provide a method for the production of an aborbent resin.
Another object of this invention is to provide a method for production of an absorbent resin which exhibits high level of absorption capacity under load as well as absorption capacity without load and, when used in a sanitary material, manifests a particularly excellent quality.